This invention relates generally to the solid phase bonding of metals, that is, the bonding of metals without the formation of any liquid phase material at the interface of the metals being bonded, and more particularly to methods for solid phase bonding one or more layers of a lead alloy and a core layer of other metal having a selected attribute of stiffness.
Composites formed of a core of titanium, austenitic stainless steel, low carbon steel, copper, aluminum or other material having suitable ductility and which can provide desired attributes of stiffness and corrosion resistance to the composite, and outer layers of lead alloys are particularly useful for various applications including batteries, ducting, x-ray shielding, chemical plants and sound attenuation. One application of particular interest is that of battery plates for lead-acid batteries.
Electric and hybrid vehicles (i.e., vehicles having auxiliary power sources such as gasoline engines) are presently considered the most practical solution to meet increasingly tighter requirements for exhaust emission from off and on road vehicles. For these applications, at present, there are a number of power sources at various stages of development and use. These include lead/acid batteries, nickel-cadmium batteries, nickel-iron batteries, alkaline batteries and sodium-sulfur batteries. Of all the power sources being developed the lead/acid batteries are considered the most reliable and are the most widely accepted. Conventional lead/acid batteries are, at present, the most widely used electrical power source for the automotive industry (cars, vans, buses and trucks), for off-road vehicles such as fork trucks and for recreational vehicles such as golf carts. The advantages of lead/acid batteries include low cost, high voltage per cell and good capacity life. The disadvantages of such batteries are their bulky size and heavy weight. Batteries may constitute a significant portion, as much as 20 to 30%, of the total weight of the vehicles. This limits the range and the usefulness of vehicles, both electric and hybrid, using such batteries.
A bi-polar design for lead/acid batteries is considered a solution to overcome the size and weight limitations of conventional lead/acid batteries. In this design, the battery is made up of stacks of bi-polar plates. Each plate consists of positive and negative lead active surfaces on opposite sides of a separator core material. The core material provides separation between the two surfaces of opposite polarity, rigidity to the composite plate and/or serves as a corrosion barrier to the battery acid, such as sulfuric acid. The bi-polar design incorporates grids on the lead surfaces on which active materials, such as lead oxide, are pasted. Ions discharged at the anode travel through the material and collect at the cathode at the opposite surface. The bi-polar battery thus operates in a sealed gas mode where oxygen produced at the positive side of each plate diffuses to the negative side of the adjacent plate where it is reduced.
The biggest barriers to the commercialization of the bi-polar lead/acid batteries are the technical difficulties of manufacturing the composite plates and the difficulty of producing the grids.
It is an object of the present invention to provide a method for producing composite materials for bi-polar plates and for forming such plates having selected shapes or pockets for reception of pasted active materials. It is desirable to form a composite by roll bonding or cladding outer layers of lead or lead alloy to a core layer of suitable material; however, according to known techniques, a preliminary deposition of a layer of lead or lead alloy on the core is required by some other method such as coating by hot-dipping in a lead-rich alloy as noted by G. Pickard and P. F. Rimmer in a paper published in Sheet Metal Industries in January of 1974 wherein it is stated that a metal-to-metal bond cannot be formed by rolling lead and steel alone (page 18). This preliminary deposition not only is time consuming but it adds undesirable cost. It is an object of the present invention to provide a method which overcomes the limitations of the prior art noted above. Yet another object of the invention is a provision of a method for bonding one or more layers of lead alloy directly to a layer of commercially pure titanium, austenitic stainless steel, low carbon steel, copper, aluminum, alloys thereof or other metal having suitable ductility and which can provide desired attributes of stiffness and corrosion resistance to the resulting composite for any of various applications.
Briefly described, in accordance with the invention, a core layer of titanium, stainless steel, copper, low carbon steel, aluminum, alloys thereof or other suitable metal is prepared for roll bonding by chemical and mechanical cleaning. The outer layers of lead or lead alloys are prepared for roll bonding by vapor degreasing or chemical cleaning. The cleaned core material is sandwiched between two or more outer layers of cleaned lead or lead alloys. The sandwiched package is passed between a pair of bonding rolls in a conventional roll bonding mill. The sandwiched package is rolled in one pass with sufficient force to reduce the package thickness by over approximately 40%, preferably by about 55% and with the outer lead or lead alloys and the core material reduced in thickness simultaneously in about the same proportion. A solid state bond is thus created in the roll bonded material between the interfaces of the core material and the lead or lead alloys. The solid state bond in the composite material may be further strengthened with a room temperature aging cycle or an elevated temperature annealing cycle.
The roll bonded material is then edge trimmed, if needed, to remove edge cracks and then continuously rolled to the desired finish gauge. The finish rolled material is then edge trimmed, if needed, and then corrugated in a conventional corrugation mill. The corrugated material is sheared to size and then used to make bi-polar battery plates.
According to a feature of the invention the differences in physical properties between the core layer and the outer lead or lead alloy layers, particularly yield strength, is minimized by softening the core material, as by annealing, and by strengthening the lead or lead alloy layers by the addition of a minor amount of calcium or antimony, more specifically, less than 1% by weight. According to another feature of the invention, the amount of tension applied to the several layers going into the roll mill may be adjusted to control the specific thickness ratios of the several layers to one another. According to yet another feature of the invention, conventional lubrication of the bonding rolls is eschewed with the bonding rolls being maintained in a dry condition.